This invention relates to polyethersulfones, and more particularly to an improved method for preparing dihydroxybiphenyl- and dihydroxybenzene-derived polyethersulfones.
Dihydroxybiphenyl-derived polyethersulfones are known in the art. A common example is the polyethersulfone derived from 4,4'-dihydroxybiphenyl and bis(4-chlorophenyl)sulfone (hereinafter sometimes "DDS"), which comprises structural units of the formula I: ##STR1##
The polyethersulfone of formula I is typically an amorphous, injection moldable resin with high solvent resistance and other desirable properties. It is most often prepared by a reaction of the DDS with an alkali metal salt of the dihydroxybiphenyl (hereinafter sometimes "biphenyl salt") at high temperature (typically at least 140.degree. C.) in solution in a dipolar aprotic solvent, as illustrated by dimethyl sulfoxide and sulfolane, which may be combined with chlorobenzene, as described, for example, in Johnson et al., J. Poly. Sci., Part A-1,5, 2375-2398 (1967). When preparation is complete, it is thus necessary to isolate the polymer by removal from the solvent, typically by precipitation with a rather large amount of anti-solvent such as chlorobenzene. This creates a mixture of organic liquids (solvent and anti-solvent) which, to make preparation economically feasible, must be separated for solvent recycle owing to the high cost of such compounds. Separation procedures are expensive and burdensome.
In an alternative method of polymer isolation, the solvent is removed by volatilization, e.g., by extrusion or film-trusion. It is preferred, however, to separate by-product alkali metal chloride salt from the polymer before volatilization is undertaken. Removal by filtration is usually impracticable because of the high viscosity of the solution and the small size of the salt crystals. Removal by dissolution in water is even less feasible, since the organic solvent is miscible with water. Thus, polymer preparation in this way is not commercially advantageous.
Another method of preparation, disclosed specifically or by analogy in U.S. Pat. Nos. 5,229,482 and 5,830,974, employs a less polar, water-immiscible solvent such as o-dichlorobenzene or anisole in combination with a phase transfer catalyst. Temperatures in the range of about 125-250.degree. C. are utilized, making it advantageous to employ a phase transfer catalyst which is resistant to high temperatures, such as a hexaalkylguanidinium halide. An advantage of this method is that the by-product salt can be removed by extraction with water, since the solvent is water-immiscible. However, such extraction is often cumbersome and inefficient since there is generally poor phase separation after introduction of water.
In various solid state methods for the preparation of aromatic polycarbonates, as disclosed, for example, in U.S. Pat. Nos. 4,948,871, 5,204,377 and 5,717,056, the normally amorphous polymer is crystallized, often by heat treatment or by contact with a solvent or non-solvent. Such crystallization operations are, however, not known to be operative with polyethersulfones.
It is of interest, therefore, to develop improved methods of preparing and isolating polyethersulfones, particularly dihydroxybiphenyl-derived polyethersulfones and the like. In particular, a method is desired which enables isolation by operations which do not include steps requiring anti-solvent precipitation or separation of phases which separate poorly.